1. Field of the Invention
This invention relates to a coating composition capable of forming hard film having excellent abrasion resistance, surface smoothness, flexibility, water resistance, heat resistance, solvent resistance, durability and adhesiveness to substrates.
2. Description of the Prior Art
Synthetic resin molded products produced from polymethyl methacrylate resin, polycarbonate resin, polyallyldiglycol carbonate resin, polystyrene resin, styrene-acrylonitrile copolymer resin (AS resin), polyvinyl chloride resin, acetate resin, acrylonitrile-butadiene-styrene copolymer resin (ABS resin), polyester resin, etc. have various advantages such as light in weight, excellent shock resistance, low cost, easy moldability, etc. as compared with glass articles and have been developed in various fields of use such as optical uses, e.g., organic plate glasses, covers for lighting fitting, optical lens, spectacle lens, light reflector, mirror and the like, decorative uses, e.g., signs, displays and the like and automotive parts, e.g., name plates, dust cover cases and the like. However, the surfaces of these synthetic resin molded products have insufficient abrasion resistance and thus the surface may be damaged by contact and collision with other articles or scratched during transportation of the molded products, installation of parts or use of the products to cause decrease in yield of the products or to damage a beautiful appearance. Especially, when the molded products are used as optical lenses such as those of cameras, magnifying glasses, etc. spectacle lenses such as fashion glasses, sun glasses, lenses for correction of eyesight, etc., window glass, decoration cases, covers, clock lenses, light reflectors, mirrors and the like. The damages to the surface cause an extreme reduction in the commercial value and renders them unusable in a short period of time. Thus, a need for improved abrasion resistance exists.
Previously, many proposals have been suggested to improve the abrasion resistance of the synthetic resin molded products. For example, there is a method which comprises coating silicone material or melamine material onto the surface of synthetic resin molded products and hardening it with heat treatment, namely, forming the so-called thermosetting type crosslink-hardened film on the surface of the synthetic resin molded products. However, since these methods are thermosetting type, not only is storage stability of the coating material unsatisfactory, but formation of crosslink-hardened film requires heating at high temperatures for a long period of time. Therefore, productivity is low and, furthermore, since the hardening reaction proceeds gradually even after the crosslinking and hardening treatment, cracks occur in the crosslink-hardened film on the products or cracks occur at the interface between the film and substrate which reduce adhesiveness to the substrate and cause inferior water resistance and weather resistance.
Another method comprises coating the surface of synthetic resin molded products with a polyfunctional acrylate or methacrylate monomer having at least 2 polymerizable ethylenically unsaturated groups in one molecule as the coating material and irradiating the coated monomer with actinic radiation to produce a crosslink-hardened film on the surface of the synthetic resin molded products by radical polymerization.
Since said polyfunctional (meth)acrylate (which means acrylate or methacrylate in this specification) monomers have excellent polymerization activity when irradiated with actinic radiation, these have been proposed as materials for quick-drying ink in U.S. Pat. Nos. 3,661,614, 3,551,311 and 3,551,246 and British Pat. No. 1,198,259. Moreover, U.S. Pat. Nos. 3,552,986, 2,413,973 and 3,770,490 propose application of these polyfunctional (meth)acrylate monomers as surface modifiers of synthetic resin molded products.
The present applicants have also found that polyfunctional (meth)acrylate monomers have excellent crosslink-hardening polymerizability and are effective as materials for forming crosslink-hardening film capable of improving abrasion resistance of the surface of synthetic resin molded products and they have made a number of proposals (Japanese Patent Publication No. 42211/63, No. 12886/64, No. 22951/64, No. 14859/64 and No. 22952/64).
As compared with the former method which comprises forming a crosslink-hardened film by heat treatment of heat-hardenable coating material, the method which comprises coating these polyfunctional (meth)acrylate monomers as a crosslink-hardenable coating material on the surface of synthetic resin molded products and irradiating with actinic radiation to form a crosslink-hardened film on the surface of the synthetic resin molded products has various merits. The storage stability of the coating material is good, the crosslink-hardened film can be produced in a short time, on the order of minute or second at room temperature because polymerization crosslink-hardening is effected by irradiation with actinic radiation and, thus, productivity is excellent, efficiency and abrasion resistance are excellent, no change of the hardened film occurs with lapse of time, water resistance, weather resistance, initial adhesiveness to the substrate are excellent, etc.
On the other hand, it is known that this method has the following problems. First, that if formation of the crosslink-hardened film by irradiation with actinic radiation after coating of the coating material on the surface of the synthetic resin molded proudcts is not carried out in an inert gas atmosphere such as nitrogen gas, carbon dioxide gas, a crosslink-hardened film having a sufficient abrasion resistance cannot be produced because the crosslink-hardening reaction is inhibited by the oxygen in the air. This is an extremely great problem in practical use. Not only does the operation become complicated, but variability occurs in performance which reduces the yield and increases the cost because it is difficult to maintain a constant, low oxygen concentration in an atmosphere. The second is that the polyfunction (meth)acrylate monomers generally have a high viscosity at room temperature and those which are most effective in improving the abrasion resistance, have a high viscosity which lowers their coating performance and limits the coating method of the coating material. The surface smoothness of the crosslink-hardened film is not adequate, the film thickness is not uniform, control of the film thickness is difficult and it is very difficult to form a thin crosslink-hardened film excellent in adhesiveness to the substrate, having good abrasion resistance, surface smoothness and uniformity of film thickness.
Thus, synthetic resin molded products having a crosslink-hardened film on the surface obtained by coating polyfunctional (meth)acrylate monomers as a coating material on the surface of synthetic resin molded products still have many problems and need to be improved and have not yet been put to practical use. A need continues to exist therefore, for a coating composition which improves the abrasion resistance of products prepared from synthetic resins.